Lead-free (K0.5Na0.5)NbO3 co-substituted with Ta5+ and Er3+: Structural, optical, dielectric and electrical properties

Abstract

The lead-free K0.5Na0.5NbO3 (KNN) ceramics co-substituted with 0.05 mol Ta5+ and x mol Er3+ ions (K, Na) (1-3x) ErxNb0.95Ta0.05O3, with x = 0.00 to 0.05, have been successfully manufactured by using the solid-state reaction. The XRD results showed that the ceramics crystalize in pure perovskite with a trace of secondary phase, for low Er levels (x ≤ 0.02). Above this rate, the presence of secondary phases became more significant. The optical band gap energy of the synthesis samples has been determined using UV–vis absorbance spectra using Tauc's relation. The Ta and Er co-substitution leads to decrease the Eg of KNN material. The grain size is found to be minimal ≈490 nm and 240 nm of sintered pellets at x = 0.02 and 0.05 of Er content respectively with a high density. The dielectric study of (K, Na) (1-3x) ErxNb0.95Ta0.05O3 ceramics was carried out and double peaks were observed in relative permittivity versus temperature data for pure KNN and KNNTa corresponding to orthorhombic-tetragonal-cubic phase transitions. While for Er doped-KNNTa samples only a broad ferroelectric-paraelectric phase transition is observed and it shifted to the high temperature with increasing Er content. This broad peak observed in dielectric data for x = 0.02 of Er content is due to inhomogeneity and will certainly open new applications in microwave dielectrics. The ac conductivity variation of KNErNTa ceramics with diverse Er concentration was analyzed according to the charge compensation mechanism. However, for KNNTa, a decrease in conductivity values is attributed to the reduction in leakage current density. Meanwhile, at x = 0.01 and 0.02 of Er concentration, Er-doped KNNTa became semi-conductor. Further investigation revealed that the resistance of grains and grains boundaries are both minimal for x = 0.02 sample which are responsible for the increase of ac conductivity. For high Er concentration ≥0.03, the RG and RGB values increase along with decrease of conductivity value related to the increase in grain size found in SEM micrographs.